Michelle L. Coote

Understanding Atom Transfer Radical Polymerization: Effect of Ligand and Initiator Structures on the Equilibrium Constants

Equilibrium constants in Cu-based atom transfer radical polymerization (ATRP) were determined for a wide range of ligands and initiators in acetonitrile at 22 degrees C. The ATRP equilibrium constants obtained vary over 7 orders of magnitude and strongly depend on the ligand and initiator structures. The activities of the Cu(I)/ligand complexes are highest for tetradentate ligands, lower for tridentate ligands, and lowest for bidentate ligands. Complexes with tripodal and bridged ligands (Me6TREN and bridged cyclam) tend to be more active than those with the corresponding linear ligands. The equilibrium constants are largest for tertiary alkyl halides and smallest for primary alkyl halides. The activities of alkyl bromides are several times larger than those of the analogous alkyl chlorides. The equilibrium constants are largest for the nitrile derivatives, followed by those for the benzyl derivatives and the corresponding esters. Other equilibrium constants that are not readily measurable were extrapolated from the values for the reference ligands and initiators. Excellent correlations of the equilibrium constants with the Cu(II/I) redox potentials and the carbon-halogen bond dissociation energies were observed.

Comment on the Correct Use of Continuum Solvent Models

The development of dielectric continuum solvent models 1,2 (CSMs) has facilitated the study of chemical reactions in the condensed phase in a computationally efficient manner. These methods have been parametrized to deliver accurate values of the free energies of solvation. which can be added to accurate values of the free energies in the gas-phase, to obtain the corresponding solution-phase free energy: G soln = G gas + ΔG solv + RTln(RT/P) (1) where the final term converts from the gas-phase standard state (defined by T and P) to the solution-phase standard state of 1 M. However, there are a growing number of recent studies that employ alternative approaches to evaluating G soln . This comment aims to highlight some of their shortcomings as well as clarify some potential points of confusion concerning the usage of these models.

Ab Initio Evaluation of the Thermodynamic and Electrochemical Properties of Alkyl Halides and Radicals and Their Mechanistic Implications for Atom Transfer Radical Polymerization

High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions.

Computational electrochemistry: prediction of liquid-phase reduction potentials

This article reviews recent developments and applications in the area of computational electrochemistry. Our focus is on predicting the reduction potentials of electron transfer and other electrochemical reactions and half-reactions in both aqueous and nonaqueous solutions. Topics covered include various computational protocols that combine quantum mechanical electronic structure methods (such as density functional theory) with implicit-solvent models, explicit-solvent protocols that employ Monte Carlo or molecular dynamics simulations (for example, Car-Parrinello molecular dynamics using the grand canonical ensemble formalism), and the Marcus theory of electronic charge transfer. We also review computational approaches based on empirical relationships between molecular and electronic structure and electron transfer reactivity. The scope of the implicit-solvent protocols is emphasized, and the present status of the theory and future directions are outlined.

The mechanism of the propagation step in free-radical copolymerisation

Abstract The propagation mechanism of free-radical copolymerisation is critically reviewed and it is concluded that the ‘explicit’ or complete penultimate model should be regarded as the base model for the majority of copolymerisation reactions. This conclusion is based on the weight of evidence from the cumulative findings of a diverse range of studies, viz. the determination of average propagation rate coefficients in co and terpolymerisation systems, moderated copolymerisation studies, solvent effects studies, ESR studies of model-radicals, and both experimental and theoretical studies of gamma-substituent effects in small-radical addition reactions. The consequences of this finding for the understanding of copolymerisation are wide-ranging. In particular, it implies that the widely published terminal-model reactivity ratios should only be regarded as adjustable parameters that do not adequately reflect the elemental processes in the copolymerisation mechanism. In addition, the studies reviewed in this paper indicate that a number of factors, including polar interactions, radical stabilisation effects, direct interactions, and entropic effects all are responsible for the penultimate unit effect in free-radical copolymerisation.

pKa Calculation of Some Biologically Important Carbon Acids - An Assessment of Contemporary Theoretical Procedures.

In this study, the aqueous pKa values for 13 neutral, 10 cationic, and 5 anionic carbon acids, including amino acids, peptides, and related species have been calculated using the high level ab initio composite procedure, G3MP2+//BMK, combined with solvation energies that were calculated using the CPCM-(UAKS/UAHF), COSMO-RS, and SM6 continuum models. The pKas were further calculated using three schemes, namely the direct method and the proton exchange method as well as the inclusion of an explicit solvent water molecule. The results of this study indicate that the direct method is unsuitable for computing the pKa of carbon acids, whereas the other two schemes perform significantly better with varying degrees of success, depending on the charge of the carbon acid. Specifically, the combination of the proton exchange scheme and CPCM-UAKS model performed particularly well for neutral species, with mean absolute deviations (MADs) of ∼1 pKa unit. The ionic species were more problematic, though the combination of the proton exchange scheme and the SM6 and CPCM-UAKS models performed reasonably well for the cationic and anionic acids, respectively. The inclusion an explicit water molecule generally improved the calculated values for anionic carbon acids.

Electrostatic catalysis of a Diels–Alder reaction

It is often thought that the ability to control reaction rates with an applied electrical potential gradient is unique to redox systems. However, recent theoretical studies suggest that oriented electric fields could affect the outcomes of a range of chemical reactions, regardless of whether a redox system is involved. This possibility arises because many formally covalent species can be stabilized via minor charge-separated resonance contributors. When an applied electric field is aligned in such a way as to electrostatically stabilize one of these minor forms, the degree of resonance increases, resulting in the overall stabilization of the molecule or transition state. This means that it should be possible to manipulate the kinetics and thermodynamics of non-redox processes using an external electric field, as long as the orientation of the approaching reactants with respect to the field stimulus can be controlled. Here, we provide experimental evidence that the formation of carbon–carbon bonds is accelerated by an electric field. We have designed a surface model system to probe the Diels–Alder reaction, and coupled it with a scanning tunnelling microscopy break-junction approach. This technique, performed at the single-molecule level, is perfectly suited to deliver an electric-field stimulus across approaching reactants. We find a fivefold increase in the frequency of formation of single-molecule junctions, resulting from the reaction that occurs when the electric field is present and aligned so as to favour electron flow from the dienophile to the diene. Our results are qualitatively consistent with those predicted by quantum-chemical calculations in a theoretical model of this system, and herald a new approach to chemical catalysis.

Mechanism of Carbon-Halogen Bond Reductive Cleavage in Activated Alkyl Halide Initiators Relevant to Living Radical Polymerization: Theoretical and Experimental Study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(•-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(•-), which is shown here to be nonexistent.

Adaptable Hetero Diels-Alder Networks for Fast Self-Healing under Mild Conditions

A novel adaptable network based on the reversible hetero Diels-Alder reaction of a cyanodithioester and cyclopentadiene is presented. Reversible between 50-120 °C, the adjustable and self-healing features of the network are evidenced via temperature dependent rheology experiments and repetitive tensile tests whereas the networks chemical structure is explored by temperature dependent (1) H MAS-NMR spectroscopy.

Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions.

High-level ab initio molecular orbital theory is used to obtain benchmark values for the ferricenium/ferrocene (Fc(+)/Fc) couple, the IUPAC recommended reference electrode for nonaqueous solution. The gas-phase ionization energy of ferrocene is calculated using the high-level composite method, G3(MP2)-RAD, and two higher-level variants of this method. These latter methods incorporate corrections for core correlation and, in the case of the highest level considered, use (RO)CCSD(T)/6-311+G(d,p) in place of (RO)CCSD(T)/6-31G(d) as the base level of theory. All methods provide good agreement with one another and the corresponding experimental values. Solvation energies have been calculated using PCM, CPCM, SMD, and COSMO-RS. Using G3(MP2)-RAD-Full-TZ gas-phase energies and COSMO-RS solvation energies, the absolute redox potentials of the Fc(+)/Fc couple have been calculated as 4.988, 4.927, and 5.043 V in acetonitrile, 1,2-dichloroethane, and dimethylsulfoxide solutions, respectively.